1. Field of the Invention
The present invention relates to heat-developable photosensitive materials. Particularly, it relates to heat-developable photosensitive materials having a layer of high sensitivity.
2. Description of the Prior Art
The photographic process using silver halide has been most widely practiced hitherto, because the photographic process using silver halide provides excellent photographic properties such as good sensitivity or gradation as compared with other photographic processes such as an electrophotographic process or a diazo photographic process. However, silver halide photosensitive materials used in this process must be subjected to development using a developer after image exposure and then subjected to various processings such as stopping, fixation, water wash or stabilization so that the areas which were not developed (hereinafter, called background) do not blacken. Accordingly, these processings are time-consuming and labor-some. Further, there are problems in that the handling of chemicals is dangerous to humans or the hands and clothes of workers in the processing room are stained at processing. Thus, it is very desired to improve the photographic process using silver halide so that the processings are carried out in a dry state without carrying out solution processing and the processed images can be preserved in a stabilized condition.
Therefore, many efforts have been made hitherto. A first attempt is the so-called combined developing-fixing process as described in, for example, U.S. Pat. No. 2,875,048, British Pat. No. 954,453 and German Pat. No. 1,163,142 wherein the development and the fixation are carried out in one processing as in the prior silver halide photographic process. A second attempt is to convert the wet processings in the silver halide photographic process into dry processings as described in, for example, German Pat. No. 1,174,159 and British Pat. Nos. 943,376 and 951,644. A third attempt is to use heat-developable photosensitive materials which contain a silver salt, for example, a silver salt of a higher fatty carboxylic acid such as silver behenate, silver saccharin or silver benzotriazole as a main photosensitive element and a catalytic amount of silver halide as described in U.S. Pat. Nos. 3,152,904, 3,457,075, 3,635,719, 3,645,739 and 3,756,829 and Canadian Pat. No. 811,677.
The present invention belongs to the third of the above described three attempts.
However, in the heat-developable photosensitive materials proposed heretofore, sufficient light sensitivity has not been obtained.
It is known by, for example, U.S. Pat. Nos. 3,457,075 and 3,761,279 that merocyanine dyes are effective as spectral sensitizers for silver halide emulsions for sensitizing heat-developable photosensitive materials. However, since all of the spectral sensitizers which are effective for silver halide emulsions are not always effective in general for heat-developable photosensitive materials, prediction of what kind of structure would be effective as a sensitizer for heat-developable photosensitive materials is difficult. Further, many heat developable photosensitive materals lack sensitivity to light in the blue range (light having a wavelength below 500 nm, hereinafter "blue light range") as described in U.S. Pat. No. 3,761,279. In the dyes described in U.S. Pat. No. 3,761,279, although sensitivity to the blue light range has been improved, the sensitivity is not sufficient. In these cases, it is not possible to predict what kind of sensitizer has high sensitivity to the blue light range.